With the advancement of computer network technology, the demand for information is increasing exponentially. Over the past decade, the fast development of Internet and the accompanying huge requirement for information transmission not only call for a more advanced fiber communication system, but also demands the development of advanced fiber communication technology. This is especially true for optical communication devices. This new demand requires new theories and cost-effective devices to support the further development of optical networks. Fortunately, the photonic integration circuits (PICs) technology developed in recent years conforms to the development of times and is opening a new era of optical networks. PICs technology is considered to be the cutting-edge and the most promising technology of optical communications. In the Silicon Valley of the United States of America, Infinera Corporation has realized the integration of a large number of complex optoelectronic integrated devices with indium phosphide and other materials, lowering the cost of optical communication while increasing its capacity. In the field of passive optical devices, Bragg grating waveguide shows an excellent property of wavelength selectivity and has been used in a variety of optical communication devices and photonic integrated devices, such as planar integrated Bragg grating waveguide filter [1] with which multiplexer/demultiplexer or filter of light signal with different wavelengths waveguide grating assisted components (OADM) [2-3], tilted waveguide grating mode converter, etc. [4] are achieved. In actual design, it is necessary to achieve different functions of the waveguide grating in a single photonic chip, which means different grating profiles have to be written individually for different functions. In particular, in order to achieve different grating directions, cycles, phase shifts, chirps and even arbitrary structures in the same chip, the traditional low-cost holographic exposure is almost impossible to actualize. Therefore, more advanced nano-fabrication technology, such as electron beam (E-Beam) lithography, is often utilized. However, the high cost and time-consuming properties of E-Beam lithography have increased the difficulty and cost of fabrication and limited its large-scale implementation.
In order to solve this practical problem, Chen Xiangfei et al first proposed an effective solution to simplify the fabrication of fiber gratings, and they called it “Reconstruction-Equivalent-Chirp (REC) technology” [5-6]. With this technology, we can fabricate a nano-scale grating structure with micron-scale precision. This method has also been successfully applied to the design and fabrication of the distributed feedback (DFB) semiconductor laser and the DFB laser array [7-9], which provides an effective solution for high performance of semiconductor laser array in photonic integration. In order to further solve the monolithic integration problem of different waveguide gratings with complex structure in the planar photonic integration and to lower the fabrication costs, based on the previous research of the applicants of this invention, the applicants propose a micro-structure quasi-phase matching (MS-QPM) technology. This technology not only provides a new method of design and fabrication of the waveguide grating with two or three dimensions, but also gives some novel grating structure and corresponding optical properties of waveguide gratings or volume gratings. For instance, we can change the grating period and even rotate the grating directions equivalently and simultaneously by sampling in the same seed grating. The REC technology is a special one-dimensional case of the micro-structure of quasi-phase matching (MS-QPM) [5]. The mathematical expression of this technology shares some similarities with the famous quasi-phase matching (QPM) described in non-linear optical materials [10,11], and therefore it can be considered as a new discovery and development of quasi-phase matching technology. In summary, this technology can achieve arbitrary shape of two or three-dimensional gratings by changing the large-scale sampling structure while keeping the seed grating period unchanged. Any of the physical achievable of two or three-dimensional grating structure can be achieved as long as the two or three dimensional gratings design is used by sampling structure with micrometer scale and uniform seed grating. With this structure we can achieve a variety of optical properties of the waveguide grating or volume grating with the fine grating structures. We only need to change the sampling structure while keep the seed grating uniform. The sampling structure size is normally a few micrometers, the implementation of this method only requires a standard holographic exposure technique with conventional photolithography technique. This greatly eases fabrication process and substantially improves productivity and product quality. The idea of two or three-dimensional sampled grating structures can be used to design new photonic devices, such as wavelength division multiplexer, which until now are array waveguide grating (AWG) and multimode interference (MMI) in the mainstream market. These existing devices have high requirement for waveguide accuracy, aside from relatively large size. Based on this two-dimensional sampling structure, combined with Bragg grating reflection principle, we can make a new compact wavelength division multiplexer. In addition, some other photonic devices such as filter without retroreflection, DFB semiconductor lasers with suppressed 0th channel resonance based on the Reconstruction-Equivalent-Chirp technology, directional coupler and power splitters of any angle, optical waveguide mode converter, any other photonic devices based on waveguide grating and the volume grating, can be achieved. We believe this method can open a new avenue and bring a new dawn to the design of planar photonic integration and other relative photonic devices.
The main idea of this invention is to propose the micro-structure of quasi-phase matching technology. Based on this technology, the target waveguide grating or volume waveguide grating with any grating shape and the corresponding photonic devices can be achieved by two or three-dimensional sampling structures using a uniform grating.